Provided you can get a signal, a mobile should cover all but the most catastrophic accidents. Even if you had a deep gash in the leg and broken bone, you could still use a modern smartphone if it has fast dial contacts on the front to alert relatives or close friends with one finger. I've had various incidents such as coming off motorcycles and cuts over the years but have never been unconscious or incapacitated enough to stop me swearing profusely. Maybe if worried you could put 999 on the front, although there's always the possibility of then accidental dialling which they wouldn't appreciate.
I always ensure I have a charged mobile when working with a chainsaw on my own. Most of the time its just cross cutting. If felling a large tree or doing something with an element of more risk like winching or moving big loads, I'd always have someone with me, even if its just the wife looking on. I find an audience makes you act more sensibly too.
Overall health is also a factor, anyone with a potential risk such as a heart condition, diabetic or simply very old should always maintain regular contact with others and tell them if wandering off on their own. My father is pushing 90 and now lives alone. Although we don't always have much to talk about I call him every day just to check he's OK, as any dutiful son should.
